This invention is related to supervisory circuits for electrical power transmission networks; and, more particularly to synchronism check relays which supervise reclosing of power circuit breakers interconnecting line and bus voltages of an AC transmission network.
Power circuit breakers are commonly provided to couple a line voltage to a bus voltage in AC transmission networks. The power circuit breaker is typically controlled by a monitoring system which, upon detection of a fault within a particular portion of the AC transmission network, commands the power circuit breaker for that particular portion to open, thereby decoupling it from the network.
It is desirable that the power circuit breaker remain in its open position until the fault has been corrected or dissipates itself. The circuit breaker should then be reclosed unless abnormal conditions such as excessive phase angle differences and/or excessive slip frequency differences between the line and bus voltages of the transmission network are present. It is desirable to provide means to determine if the phase angle and slip frequencies between the line and bus voltages are within allowable limits prior to reclosing the power circuit breaker. It is further desirable to provide means for determining the phase angle and slip frequency differences in a minimum amount of time in order to allow for high speed reclosing operations.
Typically, a synchronism check function checks the angle between the voltages on the bus and line sides of the power circuit breaker and will provide an output which enables the breaker to close if the angle is within set limits for a set period of time. The combination of angle and time settings establishes the slip cut-off frequency; that is the slip frequency above which the synchronism check function will not permit reclosing. For any given closing angle, a lower slip cut-off will result in a longer time delay. Since this time delay is introduced for all measurements, a requirement for high speed reclosing and low slip cut-off settings may result in conflict. Consequently, it is desirable to provide a synchronous check relay which permits fast reclosing while setting a reasonably low slip cut-off frequency.
Accordingly, it is an object of the present invention to provide supervisory means whose determination of reclosing of the power circuit breaker is accomplished in an relatively short period of time in order to permit fast reclosing operations.
It is another object of the present invention to provide supervisory means for monitoring phase angles and slip frequencies between bus and line voltages on each side of an open power circuit breaker in an AC power transmission network and to quickly determine that they are within predetermined limits in order to enable high speed reclosing of the breaker.
It is another object of the present invention to provide means for setting the allowable limits of the phase angle and slip frequency differences that are easily adjustable in order to accommodate requirements of various AC transmission networks.
It is a further object of the present invention to provide highly reliable devices in order to provide a highly secure supervisory means.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of the invention.